The present invention relates generally to a method for manufacturing a semiconductor device, and, more particularly, to a method for forming a device isolation film of a sub-micron scale semiconductor device.
Current semiconductor devices have attained such high integration densities that the dimensions of the device isolation regions thereof have shrunk to a sub-micron level. In such high-density semiconductor devices, a bird's beak is generated when a device isolation film is formed using a local oxidation of silicon (LOCOS) method for forming a semi-recessed field oxide film. Accordingly, device isolation is exceedingly difficult to achieve at sub-micron device geometries. Therefore, a selective polysilicon oxidation (SEPOX) method has been proposed to overcome the shortcomings and limitations of the LOCOS method.
With reference to FIGS. 1A and 1B, a typical SEPOX method will now be described. First, with reference to FIG. 1A, a thin pad oxide film 12 is formed on a semiconductor substrate 10 using a thermal oxidation process. Then, polysilicon and nitride are sequentially deposited on the pad oxide film 12, to thereby form a buffer silicon film 14 and an oxidation prevention film 16. Then, using a standard photolithographic etching process, the oxidation prevention film 16 and the buffer silicon film 14 are partially etched, to thereby form an aperture 18 in the region where a field oxide film is to be subsequently formed.
Next, with reference to FIG. 1B, using a thermal oxidation process, the exposed surfaces of the buffer silicon film 14 and the substrate 10 are selectively oxidized, to thereby form a device isolation film 20.
In accordance with the above-described SEPOX method, oxidation stresses generated in the semiconductor substrate when the device isolation film is formed are decreased, and the size of the bird's beak is reduced. However, the SEPOX method still has drawbacks. Most importantly, a bird's beak occurs at two places when the geometries of the active regions of the device fall to low sub-micron levels, effectively limiting the achievable minimum geometries of the device. More particularly, a lower bird's beak is generated between the pad oxide film 12 and the semiconductor substrate 10, as expected, but, additionally, an upper bird's beak (labelled "A" in FIG. 1B) is generated between the oxidation prevention film 16 and the buffer silicon film 14. The lower bird's beak effectively reduces the size of the active region, and the upper bird's beak degrades the reliability of the device.
These methods are not applicable to the manufacture of 64Mb and 256Mb DRAMs, since they cannot achieve isolation regions of less than about O.5 .mu.m. To overcome this limitation, a high-temperature heat treatment process is performed in a nitrogen atmosphere at a temperature of 1,150.degree. C., immediately after depositing the oxidation prevention film 16. However, this technique also presents several problems, as discussed below.
First, with extended exposure to such high temperatures, a silicon wafer tends to warp, and, in subsequent photolithographic steps, misalignment problems arise. Thus, the wafer warpage problem impedes mass production of DRAMs having a memory capacity of 64Mb or higher.
Second, nitrogen reacts with the buffer silicon film, to thereby cause nitrogen pitting on the surface of the wafer when the high-temperature heat treatment process is performed.
Third, thermal stress is generated due to a difference in the thermal expansion coefficients of the various layers which are deposited on the silicon substrate, thereby causing dislocation defects in the substrate when the high-temperature heat treatment is performed.
The generation of the lower bird's beak can be suppressed by decreasing the thickness of the pad oxide film 12 while increasing the thickness of the buffer silicon film 14. However, the generation of the upper bird's beak ("A") cannot be suppressed by varying the thickness of the pad oxide film 12 and the buffer silicon film 14.
In addition, when the upper bird's beak becomes large enough, a residue of the buffer silicon film 14 remains between the upper and lower bird's beaks, even when the oxidation prevention film 16 and the buffer silicon film 14 are removed after formation of the device isolation film 20.
To overcome the problem of an upper bird's beak which is generated when a device isolation film 20 is formed by the SEPOX method, the following two methods have been proposed. In accordance with the first method, a native oxide film formed on the buffer silicon film 14 is converted into an oxynitride film by performing an NH.sub.3 treatment after depositing the buffer silicon film 14. In accordance with the second method, a coupling reaction between the buffer silicon film 14 and the oxidation prevention film is activated by performing an N.sub.2 ion-implantation step after depositing the oxidation prevention film 16.
Fourth, if, after the buffer silicon film formation step, the manufacturing process is delayed for any reason, a native oxide film grows beyond a tolerable thickness, e.g., 15 angstroms. If the native oxide film reaches such a thickness, it becomes exceedingly difficult, if not impossible, to suppress the generation of the upper bird's beak.
Accordingly, there presently exists a need for a method for forming a device isolation film which enables reduction of an upper bird's beak without warping the wafer, and which generally overcomes the above discussed drawbacks and shortcomings of the presently available technology. The present invention fulfills this need.